The invention relates to a bone plate having a through-hole which extends from a top side to a bone-side underside of the bone plate. A lip is formed in the through-hole, said lip projecting from the lateral surface of the through-hole and extending in the circumferential direction of the through-hole.
Such bone plates are designed to be connected to a bone. They can be used for example to stabilize a bone following a fracture. To this end, the bone plate is positioned such that it extends across the fracture site and is then secured to the bone fragments. As a result, the fracture site is immobilized and the bone can heal. The bone plate can also be used for other purposes and be, for example, an element, to be connected to a bone, of an endoprosthesis.
In order to secure the bone plate to the bone, a bone screw is inserted into the through-hole, said bone screw being provided with a thread both on the shank and on the head. The shank of the screw passes so far into the bone material that the head of the screw passes into the through-hole in the bone plate. The head of the bone screw is dimensioned such that its outside diameter is larger than the smallest diameter of the plate hole, for example at the level of the material lip. As the bone screw is screwed in further, the lip is deformed such that a threaded connection is formed between the head of the bone screw and the lip in the through-hole in the bone plate. Since there is a threaded connection both between the shank of the bone screw and the bone material and between the head of the screw and the bone plate, a secure connection between the bone plate and the bone is established. Since the threaded connection between the head of the bone screw and the bone plate is formed only by a deformation process when the bone screw is screwed in, it is not necessary to screw the bone screw into the bone plate at a particular predetermined angle. Rather, the angle can be selected freely within particular limits. This gives the surgeon a high degree of flexibility during the operation.
In previous bone plates, the lip extends around the entire circumference of the through-hole. In order to establish a stable threaded connection, the screw is screwed so far into the lip until there is a threaded connection with the lip around the entire circumference of the screw head. In order to achieve this, the screw has to be screwed in an appreciable amount further after first coming into engagement with the lip, with the material of the lip being deformed to a considerable extent.
During each material deformation, there is the risk that small particles can be removed from the surface of the material. If these particles are distributed in the environment of the bone place and come into contact with the tissue, complications which hinder the progress of the healing process can occur.